Coordination of anti-epileptic inhibitory mechanisms in neocortex

Epilepsy is among the most common serious neurological conditions, affecting over a half million people in the UK, and over 50 million worldwide. Given its prevalence, it is important to ask how epileptic and normal brains differ. One hypothesis is that normal brains are protected by a particular set of inhibitory neurons, which act like circuit breakers: they fire in response to surges of activity, when the risk of a seizure is high, and prevent activity spreading out from its site of origin. The importance of this idea is that it can explain many different facets of epilepsy, from how genetic mutations give rise to seizures, right up to the nature of EEG rhythms.

Recently we have developed new ways of studying epilepsy, using the latest microscopy and electrophysiology technology to film epileptiform activity with unprecedented resolution. This work has highlighted the unusual activity patterns of certain inhibitory neurons. These cells are connected by unusual electrical contacts called gap junctions. Other evidence also points to the importance of gap junctions in epilepsy, and thus they represent a potential new target for anti-epileptic drugs. We will therefore investigate how gap junctions affect the behaviour of this important group of neurons.

Our studies will initially involve a very detailed study of the electrical properties of these unusual networks of cells, coupled by gap-junctions. These are very important baseline studies because they will show how normal brain activity spreads. By charcterizing these normal activity patterns, we can then determine how activity changes in epilepsy. To do this, we will use one of the best mouse models of epilepsy, in which mice have the same genetic mutation which has been identified as the cause of a particularly severe type of epilepsy, called Dravets syndrome. These animals, which mimic exactly real human conditions, because they derive from the exact same genetic mutations, are about the best models there are of human epilepsy. We are entering a period when such models may be created with increasing ease, yet we do not have a good way of characterizing their epilepsy. Our studies will show how to do this, by characterizing different facets of epileptic activity with exquisite detail using latest electrophyiological and microscopy techniques.

Finally we will move from mouse to humans, making use of a truly unique set of recordings taken from real people who are undergoing neurosurgical treatment for their epilepsy. These recordings have been made at a top American hospital, and have already allowed us to uncover new ways of interpreting one of the most important neurological diagnostic tools, the EEG.

This project is part of a long-running research agenda to determine what stops focussed bursts of activity spreading out to become full epileptic seizures. Once recruited to an epileptic event, pyramidal cells fire intensely on the crest of large rhythmic depolarizations. Prior to this however, is a period when they experience similarly large depolarizations but do not fire, suggestive of a powerful restraint. This inhibitory veto is a strong candidate for regulating activity at this critical juncture in the evolution of a seizure.
How is this inhibitory response to a network crisis coordinated? Undoubtedly synaptic mechanisms are involved, but an underexplored role is also likely played by gap-junctions (GJs). We hypothesize that the exact pattern of intense discharge of INs is determined by the electrotonic structure of the syncytia. We hypothesize also that the likelihood of activity spreading through the syncytial network is modulated by factors such as rising intracellular Ca2+, shifts in K+ balance and altered membrane K+ permeability. These factors will therefore determine the extent of the inhibitory surround effect. We will further explore the complementary role of discharges in syncytia of different interneuronal classes, and how these are coordinated.
To test these hypotheses, we will establish:
1. The electrotonic structure of gap-junction coupled, interneuronal syncytia
2. How ionic concentrations and neuromodulators influence syncytial behaviour
3. How syncytial behaviour is altered in transgenic mouse models of myoclonic epilepsy.
4. Whether "signature" electrophysiological features of syncytia activity can be seen in human recordings.

Our research impacts directly on one of the most common serious neurological conditions: epilepsy. The WHO estimates that, worldwide, more than 50 million people are diagnosed with epilepsy; to put this in context, about half this number suffer from any form of dementia. Over the last 8 years, we have developed sophisticated new approaches to understanding various simple in vitro animal models of epileptic propagation, but importantly, our recent collaborations with Dr Catherine Schevon have shown clearly that these studies have direct relevance to epilepsy in humans. Furthermore, our detailed understanding of the spatial pattern of activity during in vitro epileptiform discharges indicated a potential pitfall for EEG localization of seizures. We have now confirmed this from human recordings, and this will lead to a revision of how epileptic activity is localized for neurosurgical procedures. Other parallel studies are seeking to translate our basic research findings to clinical practice by developing visual tests for assessing seizure risk. These studies provide a clear proof of principle showing how simple animal research impacts directly on clinical practice.

We also outline a comprehensive approach to characterizing epileptic phenotypes in transgenic mice. This can form a template for all future epilepsy studies on such mice, and further impact on related studies examining pharmacological approaches to managing epilepsy in these animals. AT is a core member of an application to form an MRC Mouse Network Research Consortium advising on the generation and research of transgenic mice. These initiatives will impact on pharmaceutical companies designing and testing novel anti-epileptics. It is notable that the main focus of our proposal, gap junctions, are not currently targeted by any anti-epileptic medication.